In conveyors of this kind, it is known to reduce the friction between the walls of the pump housing, the pump screw and the material being handled by constructing the parts of the two screws in mutual engagement with an increasing pitch, i.e. increasing distance between the windings, in the pumping chamber. In this way, it is possible to handle very lumpy and very viscous materials. A construction of this kind is e.g. known from WO 98/35135.
U.S. Pat. No. 4,043,002 describes a construction in which the pumping screw may extend into the charging chamber. The purpose of this extension of the pumping screw into the charging chamber is to capture material and carry it into the pumping chamber. In connection with very lumpy and very viscous materials this may lead to increased friction in the pumping chamber due to overfilling.